1. Field of the Invention
The invention relates to a method of addressing a plasma display panel. More particularly, the invention relates to the coding of the grey levels of a type of panel with separate addressing and sustaining.
2. Discussion of Prior Art
Plasma display panels, called hereafter PDPs, are flat-type display screens. There are two large families of PDPs, namely PDPs whose operation is of the DC type and those whose operation is of the AC type. In general, PDPs comprise two insulating tiles (or substrates), each carrying one or more arrays of electrodes and defining between them a space filled with gas. The tiles are joined together so as to define intersections between the electrodes of the said arrays. Each electrode intersection defines an elementary cell to which a gas space corresponds, which gas space is partially bounded by barriers and in which an electrical discharge occurs when the cell is activated. The electrical discharge causes an emission of UV rays in the elementary cell and phosphors deposited on the walls of the cell convert the UV rays into visible light.
In the case of AC-type PDPs, there are two types of cell architecture, one called a matrix architecture and the other called a coplanar architecture. Although these structures are different, the operation of an elementary cell is substantially the same. Each cell may be in the ignited or xe2x80x9conxe2x80x9d state or in the extinguished or xe2x80x9coffxe2x80x9d state. A cell may be maintained in one of these states by sending a succession of pulses, called sustain pulses, throughout the duration over which it is desired to maintain this state. A cell is turned on, or addressed, by sending a larger pulse, usually called an address pulse. A cell is turned off, or erased, by nullifying the charges within the cell using a damped discharge. To obtain various grey levels, use is made of the eye""s integration phenomenon by modulating the durations of the on and off states using subscans, or subframes, over the duration of display of an image.
In order to be able to achieve temporal ignition modulation of each elementary cell, two so-called xe2x80x9caddressing modesxe2x80x9d are mainly used. A first addressing mode, called xe2x80x9cAddressing While Displayingxe2x80x9d (AWD), consists in addressing each row of cells while sustaining the other rows of cells, the addressing taking place row by row in a shifted manner. A second addressing mode, called xe2x80x9cAddressing and Display Separationxe2x80x9d (ADS), consists in addressing, sustaining and erasing all of the cells of the panel during three separate periods. For more details concerning these two addressing modes, a person skilled in the art may, for example, refer to U.S. Pat. Nos. 5,420,602 and/or 5,446,344.
FIG. 1 shows the basic time division of the ADS mode for displaying an image. The total display time Ttot of the image is 16.6 or 20 ms, depending on the country. During the display time, eight subscans SB1 to SB8 are effected so as to allow 256 grey levels per cell, each subscan making it possible for an elementary cell to be xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d for an illumination time Tec which is a multiple of a value To. Hereafter, reference will be made to an illumination weight p, where p corresponds to an integer such that Tec=p.To. The total duration of a subscan comprises an erasure time Tef, an address time Ta and the illumination time Tec specific to each subscan. The address time Ta can also be decomposed into n times an elementary time Tae, which corresponds to the addressing of one row. Since the sum of the illumination times Tec needed for a maximum grey level is equal to the maximum illumination time Tmax, we have the following equation: Ttot=m.(Tef+n.Tae)+Tmax, in which m represents the number of subscans. FIG. 1 corresponds to a binary decomposition of the illumination time.
One problem is the creation of false contouring which stems from the proximity of two areas whose grey levels are very close but whose illumination times are decorrelated. The worst case, in the example in FIG. 1, corresponds to a transition between the levels 127 and 128. This is because the grey level 127 corresponds to an illumination for the first seven subscans SB1 to SB7, while the level 128 corresponds to the illumination of the eighth subscan SB8. Two areas of the screen placed one beside the other, having the levels 127 and 128, are never illuminated at the same time. When the image is static and the observer""s eyes do not move over the screen, temporal integration takes place relatively well (if any flicker effect is ignored) and two areas with relatively close grey levels are seen. On the other hand, when the two areas move over the screen (or the observer""s eyes move), the integration time slot changes screen area and is shifted from one area to another for a certain number of cells. The shift in the eye""s integration time slot from an area of level 127 to an area of level 128 has the effect of integrating so that the cells are off over the period of one frame, which results in the appearance of a dark contour of the area. Conversely, shifting the eye""s integration time slot from an area of level 128 to an area of level 127 has the effect of integrating so that the cells are lit to the maximum over the duration of one frame, which results in the appearance of a light contour of the area (which is less perceptible than the dark contour). This phenomenon is accentuated when the display works with pixels consisting of three (red, green and blue) elementary cells, since the contouring may be coloured.
The phenomenon of contouring occurs at all level transitions where the switched illumination weights correspond to different temporal distribution groups. Switchings of high weight are more annoying than switchings of low weight because of their magnitude. The resulting effect may be perceptible to a greater or lesser extent depending on the switched weights and on their positions. Thus, the contouring effect may also occur with levels that are quite far apart (for example 63-128, but it is much less shocking for the eye as it then corresponds to a very visible level (or colour) transition.
To remedy the problem of contouring, one solution consists in breaking up the high illumination weights so as to reduce the visual effects of the high-weight transitions. FIG. 2 shows a solution in which 10 subscans are used, thereby resulting in an overall reduction in brightness of the panel. The maximum illumination time Tmax is then approximately 30% of the total image display time and the erasure and address time is about 70%.
The use of 10 subscans, as shown in FIG. 2, does not allow there to be perfect correction of the false contouring effect and requires an increase in the number of subscans. However, increasing the number of subscans creates a brightness-reduction problem.
In order to elevate this brightness reduction, it is known to use subscans common to two rows of the panel, thereby allowing the total number of subscans to be increased without reducing the actual image display time. FIG. 3 shows a distribution over 11 subscans, the low-weight subscans (weights 1 and 2) of which are common to two rows. The use of subscans common to two rows has the effect of dividing the address time of these subscans by two. The use of two common subscans makes it possible to use an additional subscan while maintaining a constant overall address time. But this creates a loss-of-resolution problem with the low weights.
To remedy the loss of resolution and to increase the number of common subscans, one solution consists in using a code with multiple representations. FIG. 4 shows a 12-subscan distribution, 4 of which are common to two adjacent rows. The multiple representation is based on the fact that there are several ways of coding a grey level. The coding of two adjacent grey levels is accomplished by using the coding which minimizes the error as far as possible. However, if the number of common subscans is increased, there is still a loss of resolution.
European Application EP-A-0 945 846 has disclosed a coding system which minimizes the error due to the simultaneous scanning of several pairs of rows with the aid of a code with multiple representation. FIG. 5 shows an example of coding over 14 subscans, the display time of which corresponds to about 10 subscans. In the example in FIG. 5, eight subscans of weight 1, 2, 4, 7, 13, 17, 25 and 36 are common to two rows at the same time, six subscans of weight 5, 10, 20, 30, 40 and 45 being specific to each row. The resolution error is minimized by rounding the difference between two adjacent grey levels so that the error is always equal to xc2x11.
The coding in FIG. 5 may appear ideal since the number of common subscans is very high. However, the use of many common subscans results in a coding difference error. In the example in FIG. 5, the sum of the weights associated with the subscans specific to each row is equal to 150. This means that when two adjacent cells are addressed simultaneously and the difference between the grey levels is greater than 150, an error then arises during display, which occurs on average on 1% of the dots of a video image.
The object of the invention is to propose a system for encoding grey levels which makes it possible to reduce the problems of contouring by increasing the number of subscans using subscans common to several rows, thereby remedying the error due to the difference between the grey levels of simultaneously scanned cells.
The invention is a method for displaying a video image on a display device comprising a plurality of cells in which each cell is illuminated for an illumination time by means of a plurality of subscans each having a specific duration associated with an illumination weight. The subscans are distributed as first and second subscans. The first subscans are addressed for each row of the panel. The second subscans are addressed simultaneously at row groupings having a number of rows which varies according to the image displayed.
To obtain the highest image quality, several possible ways of grouping the rows are evaluated and then the grouping which minimizes the display errors is chosen.
To reduce the address time, several possible ways of grouping the rows are evaluated and then the possible grouping which has the most rows is chosen.
To minimize the rounding errors in such a method, the illumination weights associated with the first subscans are multiples of three.
The invention also relates to a display device comprising a plurality of cells organized in rows and columns, each cell being illuminated over a display period for a time proportional to a grey level by means of a plurality of subscans, each subscan having an address time during which the rows are addressed in succession, characterized in that it includes means for addressing the rows by row grouping, the number of rows of which varies according to the image to be displayed.
More particularly, the display device is a plasma display panel comprising a plurality of discharge cells.